121 research outputs found
Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS
We report on the detection of vinyl cyanide (CH2CHCN), cyanomethyl radical
(CH2CN), methylacetylene (CH3CCH) and thioformaldehyde (H2CS) in the C-rich
star IRC +10216. These species, which are all known to exist in dark clouds,
are detected for the first time in the circumstellar envelope around an AGB
star. The four molecules have been detected trough pure rotational transitions
in the course of a 3 mm line survey carried out with the IRAM 30-m telescope.
The molecular column densities are derived by constructing rotational
temperature diagrams. A detailed chemical model of the circumstellar envelope
is used to analyze the formation of these molecular species. We have found
column densities in the range 5 x 10^(12)- 2 x 10^(13) cm^(-2), which
translates to abundances relative to H2 of several 10^(-9). The chemical model
is reasonably successful in explaining the derived abundances through gas phase
synthesis in the cold outer envelope. We also find that some of these
molecules, CH2CHCN and CH2CN, are most probably excited trough infrared pumping
to excited vibrational states. The detection of these species stresses the
similarity between the molecular content of cold dark clouds and C-rich
circumstellar envelopes. However, some differences in the chemistry are
indicated by the fact that in IRC +10216 partially saturated carbon chains are
present at a lower level than those which are highly unsaturated, while in
TMC-1 both types of species have comparable abundances.Comment: 9 pages, 5 figures; accepted for publication in A&
A new infrared band in the Interstellar and Circumstellar Clouds: C_4 or C_4H?
We report on the detection with the Infrared Space Observatory (ISO) of a
molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and
in Sgr B2. Taking into account the chemistry of these objects the most
likelihood carrier is a carbon chain. We tentatively assign the band to the
nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been
derived in matrix experiments (Withey et al. 1991). An alternate carrier might
be C_4H, although the frequency of its lowest energy vibrational bending mode,
nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived
maximum abundance is nearly similar to that found for C_3 in the interstellar
and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence,
tetra-atomic carbon could be one of the most abundant carbon chain molecules in
these media.Comment: 11 pages, 1 figure, accepted in ApJ Letter
Laboratory And Astronomical Detection Of The Negative Molecular Ion C3N-
The negative molecular ion C3N- has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s(-1) or better in equivalent radial velocity. Four transitions of C3N- were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C3N- is 0.5% that of C3N, or approximately 20 times greater than that of C4H- relative to C4H. The C3N- abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C3N- relative to C3N (< 0.8%) and a limit for C4H- relative to C4H (< 0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration ofNRFKorean government MEST 2012R1A1A1014646, 2012M4A2026720Southeast Physics Network (SEP-Net)Science and Technology Facilities Council ST/F002858/1, ST/I000976/1Swedish Research Council 2009-4088U.S. NSF AST-0708176, AST-1009799NASA NNX07AH09G, NNG04G177G, NNX11AE09GChandra grant SAO TM8-9009XBiochemistr
Molecular Line Observations of Carbon-Chain-Producing Regions L1495B and L1521B
We present the first comprehensive study on physical and chemical properties
of quiescent starless cores L1495B and L1521B, which are known to be rich in
carbon-chain molecules like the cyanopolyyne peak of TMC-1 and L1521E. We have
detected radio spectral lines of various carbon-chain molecules such as CCS,
CS, CH, HCN, and HCN. On the other hand, the NH
lines are weak and the NH lines are not detected. According to our
mapping observations of the HCN, CCS, and CS lines, the dense cores
in L1495B and L1521B are compact with the radius of 0.063 and 0.044 pc,
respectively, and have a simple elliptical structure. The distributions of CCS
seem to be different from those of well-studied starless cores, L1498 and
L1544, where the distribution of CCS shows a shell-like structure. Since the
HCO, HNC, and CS lines are detected in L1495B and
L1521B, the densities of these cores are high enough to excite the NH and
NH lines. Therefore, the abundances of NH and NH
relative to carbon-chain molecules are apparently deficient, as observed in
L1521E. We found that longer carbon-chain molecules such as HCN and
CH are more abundant in TMC-1 than L1495B and L1521B, while those of
sulfur-bearing molecules such as CS, CCS, and CS are comparable.
Both distributions and abundances of the observed molecules of L1495B and
L1521B are quite similar to those of L1521E, strongly suggesting that L1495B
and L1521B is in a very early stage of physical and chemical evolution.Comment: 19 pages, 6 figures, accepted to The Astrophysical Journa
Laboratory Measurement of the Pure Rotational Transitions of the HCNH+ and its Isotopic Species
The pure rotational transitions of the protonated hydrogen cyanide ion,
HCNH+, and its isotopic species, HCND+ and DCND+, were measured in the 107 -
482 GHz region with a source modulated microwave spectrometer. The ions were
generated in the cell with a magnetically confined dc-glow discharge of HCN
and/or DCN. The rotational constant B0 and the centrifugal distortion constant
D0 for each ion were precisely determined by a least-squares fitting to the
observed spectral lines. The observed rotational transition frequencies by
laboratory spectroscopy and the predicted ones are accurate in about 30 to 40
kHz and are useful as rest frequencies for astronomical searches of HCNH+ and
HCND+.Comment: 14 pages in TeX, 1 figures in JPE
Respuesta sísmica de suelos con histéresis y viscosidad
Un problema importante de ingeniería sísmica es la respuesta de depósitos estratificados de suelos cuando se encuentran sometidos a la acción del terremoto. El problema puede ser directo o inverso, según se pretenda obtener el movimiento en superficie cuando el fondo es solicitado por un sismo dado o, lo que es muy común en la técnica de análisis sísmico, se pretende realizar la deconvolución de un movimiento en superficie hasta una profundidad determinada con objeto de realizar a posteriori un análisis
de interacción terreno-estructura. El problema es bien conocido así como sus dificultades relacionadas principalmente con el carácter no lineal del suelo y sus propiedades de amortiguamiento
The essential signature of a massive starburst in a distant galaxy
Observations of carbon monoxide (CO) emission in high redshift (z>2) galaxies
indicate the presence of large amounts of molecular gas. Many of these galaxies
contain an active galactic nucleus (AGN) powered by accretion of gas onto a
supermassive black hole, and a key question is whether their extremely high
infrared luminosities result from the AGN, or from bursts of massive star
formation (associated with the molecular gas), or both. In the Milky Way,
high-mass stars form in the dense cores of interstellar molecular clouds; gas
densities are n(H2)>105 cm-3 in the cores. Recent surveys show that virtually
all galactic sites of high-mass star formation have similarly high densities.
The bulk of the cloud material traced by CO observations is at a much lower
density. In galaxies in the local Universe, the HCN(J=1-0) line is an effective
tracer of the high-density molecular gas. Here we report observations of HCN
emission in the early Universe from the infrared luminous 'Cloverleaf' quasar
(at a redshift z=2.5579). The HCN line luminosity indicates the presence of 10
billion solar masses of very dense gas, an essential feature of an immense
starburst that contributes, together with the AGN it harbors, to its high
infrared luminosity.Comment: PDF pape
Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
In this paper we study the oxygen chemistry in the C-rich circumstellar
shells of IRC+10216. The recent discoveries of oxygen bearing species (water,
hydroxyl radical and formaldehyde) toward this source challenge our current
understanding of the chemistry in C-rich circumstellar envelopes. The presence
of icy comets surrounding the star or catalysis on iron grain surfaces have
been invoked to explain the presence of such unexpected species. This detailed
study aims at evaluating the chances of producing O-bearing species in the
C-rich circumstellar envelope only by gas phase chemical reactions. For the
inner hot envelope, it is shown that although most of the oxygen is locked in
CO near the photosphere (as expected for a C/O ratio greater than 1), some
stellar radii far away species such as H2O and CO2 have large abundances under
the assumption of thermochemical equilibrium. It is also shown how non-LTE
chemistry makes very difficult the CO-->H2O,CO2 transformation predicted in
LTE. Concerning the chemistry in the outer and colder envelope, we show that
formaldehyde can be formed through gas phase reactions. However, in order to
form water vapor it is necessary to include a radiative association between
atomic oxygen and molecular hydrogen with a quite high rate constant. The
chemical models explain the presence of HCO+ and predict the existence of SO
and H2CS (which has been detected in a 3 mm line survey to be published). We
have modeled the line profiles of H2CO, H2O, HCO+, SO and H2CS using a
non-local radiative transfer model and the abundance profiles predicted by our
chemical model. The results have been compared to the observations and
discussed.Comment: 20 pages, 9 figures, accepted for publication in the Astrophysical
Journa
Dust and gas in luminous infrared galaxies - results from SCUBA observations
We present new data taken at 850 m with SCUBA at the JCMT for a sample
of 19 luminous infrared galaxies. Fourteen galaxies were detected. We have used
these data, together with fluxes at 25, 60 and 100 m from IRAS, to model
the dust emission. We find that the emission from most galaxies can be
described by an optically thin, single temperature dust model with an exponent
of the dust extinction coefficient () of
. A lower is required to model the dust
emission from two of the galaxies, Arp 220 and NGC 4418. We discuss various
possibilities for this difference and conclude that the most likely is a high
dust opacity. In addition, we compare the molecular gas mass derived from the
dust emission, , with the molecular gas mass derived from the CO
emission, , and find that is on average a factor 3 higher than
.Comment: 10 pages, 6 figures, latex, with MN-macros, accepted by MNRAS -
revised version (changed flux values for some galaxies
A New Probe of Dense Gas at High Redshift: Detection of HCO+(5-4) Line Emission in APM 08279+5255
We report the detection of HCO+(5-4) emission from the Broad Absorption Line
(BAL) quasar APM08279+5255 at z=3.911 based on observations conducted at the
IRAM Plateau de Bure interferometer. This represents the first detection of
this molecular ion at such a high redshift. The inferred line luminosity,
uncorrected for lensing, is L'(HCO+)=(3.5+-0.6)x10^10 Kkms^-1pc^2. The HCO+
J=5-4 source position coincides within the errors with that reported from
previous HCN J=5-4 and high-J CO line observations of this quasar. The HCO+
line profile central velocity and width are consistent with those derived from
HCN. This result suggests that HCO+(5-4) emission comes roughly from the same
circumnuclear region probed by HCN. However, the HCN(5-4)/HCO+(5-4) intensity
ratio measured in APM08279+5255 is significantly larger than that predicted by
simple radiative transfer models, which assume collisional excitation and equal
molecular abundances. This could imply that the [HCN]/[HCO^+] abundance ratio
is particularly large in this source, or that the J=5 rotational levels are
predominantly excited by IR fluorescent radiation.Comment: Accepted for publication in ApJ Letters, May 2
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